1. Field of the Invention
The present invention relates to an inspection system for wafers, masks, and/or reticles. The inspection system can include an array of TDI sensor modules with localized circuitry for driving and signal processing.
2. Related Art
Time delay integration (TDI) is an imaging process that produces a continuous image of a moving two-dimensional object. In a TDI system, image photons are converted to photocharges in an array of pixels. As the object is moved, the photocharges are shifted from pixel pixel down the sensor, parallel to the axis of movement. By synchronizing the photocharge shift rate with the velocity of the object, the TDI can integrate a signal intensity at a fixed position on the moving object to generate the image. The total integration time can be regulated by changing the speed of the image motion and providing more/less pixels in the direction of the movement. TDI inspection systems can be used for inspecting wafers, masks, and/or reticles.
A conventional TDI sensor includes a large array of photo sensor elements (e.g. charge-coupled devices (CCDs)) formed as a grid. For example, a conventional TDI sensor could be formed in a 2048×256 array of photo sensor elements. Exemplary, conventional TDI sensors are described in U.S. Pat. No. 4,580,155, which issued to Tsoi et al. on Apr. 1, 1986, U.S. Pat. No. 4,280,141, which issued to McCann on Jul. 21, 1981, and U.S. Pat. No. 4,382,267, which issued to Angle on May 3, 1983.
To achieve higher sensitivity than can be provided by using a conventional TDI sensor, U.S. Pat. No. 7,227,984, which issued to Cavan on Jun. 5, 2007, arranges a plurality of TDI pixels in a sub-pixel offset pattern. FIG. 1 illustrates a simplified, interleaved TDI sensor 100 including two sensor arrays 101 and 102 that are formed in an interleaved pattern. This sensor interleaving can advantageously increase the resolution and the anti-aliasing capability of a TDI inspection system. Each sensor array includes a plurality of hexagon-shaped pixels 103, wherein each sensor array is offset both in the vertical direction and the horizontal direction relative to an adjacent sensor array. Note that each offset is a sub-pixel distance, i.e. less than one pixel.
At increasingly smaller technology nodes, it is desirable for the image to be significantly magnified, thereby facilitating defect detection. At the same time, faster inspections are being requested, despite the increasing complexity of the wafers/masks/reticles being inspected. To accomplish these goals, the size of the TDI sensor arrays has increased.
Unfortunately, the yield associated with TDI sensor arrays decreases significantly with increases in array size. Moreover, larger TDI sensor arrays also have correspondingly larger drivers, which require more current. Additionally, the analog readout from these large sensor arrays requires dense signal trace routing and large complicated printed circuit boards. The dense signal routing increases the possibility of signal crosstalk, which can decrease the signal-to-noise ratio (SNR). Moreover, high-brightness illumination is required to provide an intense, uniform field of illumination at the inspection surface plane. The decreased yield as well as the increased driving, processing, and illumination requirements can significantly increase system resource and component costs.
Therefore, a need arises for a TDI-based inspection system that employs smaller TDI devices while mitigating the driving, processing, and illumination difficulties.